The invention relates to chain saw-mounted grinding devices for automatically grinding a saw chain while the saw chain is being driven.
A majority of chain saws have chains, the cutter links of which have one cutter and one depth gauge for limiting the cutting depth and the risk of uncontrollable kick-back and impacts. These chains are sharpened by filing or grinding the front rake face of the cutter. Since the rear clearance face of the cutter has a certain small clearance angle this will lower the edge, requiring lowering also of the depth gauge by filing, which is difficult to do with sufficient precision.
Special chains have been previously suggested, designed for automatic resharpening and height adjustment of the depth gauge with high precision by grinding devices without removing the chain from the chain saw. According to the U.S. Pat. No. 3,147,644 and U.S. Pat. No. 3,502,122 and others, a grinding device has been located immediately behind the drive sprocket of the chain, and will grind both the cutter and the depth gauge simultaneously as the chain is running around the drive sprocket. Relative to the cutter link, the grinding device will follow a circular path, the highest point of which is located where a radius from the center of the drive sprocket forms a 90 degree angle with a line passing through both of the cutter link-connecting rivets R (see FIG. 3). With normal links and sprockets, that radius and line intersect at the center of the link, as shown in FIG. 3.
That is, in grinding devices according to the prior art, the cutter links run with both of their connecting rivets R spaced at the same distance from the sprocket center (21), and as shown in FIG. 3 the radius (19) extending at 90 degrees to a line (20) passing through the rivets will pass through the center of the link. This means that in order to grind the depth gauge (31) lower than the cutter (32) that corresponds to the desired chip thickness, the depth gauge (31) will have to be located very far forward, and the cutting edge of the cutter (32) located close to the radius (19) at the center of the link.
To keep the depth gauge lower than the cutter edge by an amount that corresponds to the desired cutting depth, the depth gauge must be located at a greater distance from this radius than is the cutter edge, and to get a positive clearance angle the cutting edge must be located behind the radius, as is shown by the U.S. Pat. No. 3,147,644 and U.S. Pat. No. 3,170,497 and others. For hand-held chain saws a positive clearance angle is necessary since the available feed force is limited, which means that even with a small diameter of the drive sprocket the distance between the depth gauge and the cutter edge must be rather large. This increases the risk for kick-back and violent jerks when sawing thin branches or when a tree moves. Safety requirements have thus excluded such grinding devices from the market.
For vehicle-born chain saws the problems are different. The drive sprocket diameter must be kept rather large to keep the motor velocity and the sprocket wear low. To utilize the motor power the cutting depth should be larger than with hand-held chain saws, which would require a very long distance between the depth gauge and the cutter. In actual cases the depth gauge would have to be situated far in front of the front rivet, (see FIG. 3 and U.S. Pat. No. 3,170,497), which in turn might cause severe vibration and instability in service.